Imaging technology is the science of converting an image to a representative signal. Imaging systems have broad applications in many fields, including commercial, consumer, industrial, medical, defense, and scientific markets. Most image sensors are silicon-based semiconductor devices that employ an array of pixels to capture light, with each pixel including some type of photodetector (e.g., a photodiode or photogate) that converts photons incident upon the photodetector to a corresponding charge. CCD (charge coupled device) and CMOS (complementary metal oxide semiconductor) image sensors are the most widely recognized and employed types of semiconductor based image sensors.
The ability of an image sensor to produce high quality images depends on the light sensitivity of the image sensor which, in-turn, depends on the quantum efficiency (QE) and optical efficiency (OE) of its pixels. Image sensors are often specified by their QE, or by their pixel QE, which is typically defined as the efficiency of a pixel's photodetector in converting photons incident upon the photodetector to an electrical charge. A pixel's QE is generally constrained by process technology (i.e., the purity of the silicon) and the type of photodetector employed (e.g., a photodiode or photogate). Regardless of the QE of a pixel, however, for light incident upon a pixel to be converted to an electrical charge, it must reach the photodetector. With this in mind, OE, as discussed herein, refers to a pixel's efficiency in transferring photons from the pixel surface to the photodetector, and is defined as a ratio of the number of photons incident upon the photodetector to the number of photons incident upon the surface of the pixel.
At least two factors can significantly influence the OE of a pixel. First, the location of a pixel within an array with respect to any imaging optics of a host device, such as the lens system of a digital camera, can influence the pixel's OE since it affects the angles at which light will be incident upon the surface of the pixel. Second, the geometric arrangement of a pixel's photodetector with respect to other elements of the pixel structure can influence the pixel's OE since such structural elements can adversely affect the propagation of light from the pixel surface to the photodetector if not properly configured. The latter is particularly true with regard to CMOS image sensors, which typically include active components, such as reset and access transistors and related interconnecting circuitry and selection circuitry within each pixel. Some types of CMOS image sensors further include amplification and analog-to-digital conversion circuitry within each pixel.
Some efforts have been made to optically model the arrangement of the photodetector with respect to other elements of the pixel structure in order to predict how incident light will travel through a pixel and thereby determine a pixel configuration that will allow the most light incident on the pixel to reach the photodetector. However, such efforts have generally relied on standard geometrical optics, such as ray tracing techniques, which do not accurately model the behavior of light at image sensor scales. This is particularly true as technology scales and pixel elements become commensurate with the wavelength of light, such as CMOS image sensors implemented in a deep sub-micron technology.